It has been known that a surfactant has both of a hydrophilic part and a lipophilic part in one molecule, and exhibits various activities such as wash, dispersion, antirust, anticorrosive, moistening, penetration, bubble formation, emulsifying, solubilization and antistat. A surfactant is used for emulsifying and dispersing an insoluble component in water by utilizing the above-described activities in a various use such as a cosmetic product, a pharmaceutical product and an ink.
For example, a surfactant exhibits the function by forming an aggregate of molecules thereof in water and enclosing a lipophilic component in the aggregate. In order to form such an aggregate, it is necessary to adjust the concentration of a surfactant to a certain level. The minimum concentration of a surfactant to form the aggregate is referred to as CMC: critical micelle concentration. Each surfactant has a particular CMC value.
In general, it is preferred that an amount of a surfactant to be used is small in terms of environmental impact and economic efficiency. For example, a synthetic surfactant such as sodium linear alkylbenzene sulfonate and sodium dodecyl sulfate is utilized for various products and uses, since a synthetic surfactant can be inexpensively produced. However, a surfactant may cause various environmental damage, since a surfactant has a function to homogenize a hydrophilic component and a lipophilic component. It is therefore preferred to reduce an amount of a surfactant to be used as much as possible and not to release a surfactant to the environment. In addition, a certain surfactant, particularly a synthetic anionic surfactant, easily penetrates dermal tissue and mucous membrane to give harm to a living body. Accordingly, a surfactant having lower critical micelle concentration is considered to be better one, since such a surfactant fulfills a function even in a lower concentration and may be blended to be a composition more freely.
Thus, a surfactant which exhibits low critical micelle concentration has been developed, and it has also been studied that whole critical micelle concentration is reduced by using a plurality of surfactants. For example, it is described in Non-patent Document 1 that critical micelle concentration is reduced by mixing sophorolipid and sodium dodecylbenzenesulfonate in comparison with the case of sodium dodecylbenzenesulfonate only. Sophorolipid is one kind of biosurfactant, which is a natural surfactant produced by microorganism.
It is described in Non-patent Document 2 that the critical micelle concentration of sodium dodecyl sulfate may be changed by mixing sodium dodecyl sulfate with a long-chain alkanoyl-N-methylglucamide, i.e. MEGA-8 to MEGA-10.
It is described in Non-patent Document 3 that a plurality of surfactants are mixed to be used.